Optical transmission system

ABSTRACT

An optical transmission system which provides bandwidth restricted optical signal comprises an input terminal ( 10 ) for accepting an electrical binary signal, an amplifier ( 12 ) for amplifying said electrical binary signal to the level requested for operating an electrical-optical converter ( 16 ) such as a Mach Zehnder light modulator, a bandwidth restriction means ( 14 ) which is for instance a low pass filter for restricting bandwidth of said electrical binary signal, and an electrical-optical conversion means ( 16 ) such as a Mach Zehnder light modulator for converting electrical signal to optical signal. Because of the location of the low pass filter ( 14 ) between an output of the amplifier ( 12 ) and the Mach Zehnder light modulator ( 16 ), the amplifier ( 12 ) may operate in saturation region to provide high level output signal enough for operating the Mach Zehnder light modulator, and a signal shaped by the low pass filter ( 14 ) is applied to the Mach Zehnder light modulator ( 16 ) with excellent waveform. The invention is useful for long distance, large capacity and low cost optical transmission system.

BACKGROUND OF THE INVENTION

The present invention relates to an optical transmission system, inparticular, relates to such a system which generates high quality, highrate and bandwidth restricted optical signal free from degradation ofsignal quality in a transmission line.

In an optical transmission system, a large amount of transmissioncapacity is intended by using high-bit-rate channel, and wavelengthdivision multiplexing. In general, the higher a channel bit rate is, themore severe the effect of chromatic dispersion in an optical fiber is,and possible distance for transmission is shortened in proportional tosquare of channel bit rate.

In order to decrease the effect of chromatic dispersion due todifference of group velocity depending upon wavelength, the use ofbandwidth restricted code such as an optical duobinary transmissionsystem is useful as described in K. Yonenaga and S. Kuwano, IEEE J.Lightwave Technol., Vol. 15, No. 8, 1997.

FIG. 21 shows a block diagram of a prior optical duobinary transmitter.An input binary signal to be transmitted is applied to an input terminal10, then, to a precoder 32 which effects code conversion, through aninverter 30 which inverts a binary signal. The precoder 32 includes anexclusive OR circuit 32 a and a one bit delay circuit 32 b as shown inFIG. 21. An output logic signal of the precoder 32 is kept when an inputlogic signal is 0, and an output logic signal is inverted when an inputlogic signal is 1. An output of the precoder 32 is applied to adifferential distribution circuit 34 which provides a pair of NRZ(non-return to zero) signals in differential form. Each of the pair ofNRZ signals is converted to a ternary duobiary signal by a low passfilter 100-1 or 100-2 which has 3 dB cut-off frequency approximate atthe ¼ frequency of signal clock frequency. A filter 100-1 or 100-2 whichoperates above is called a duobinary filter. An electrical-opticalconverter 110 is for instance implemented by a Mach Zender intensitymodulator (MZ) of dual electrode drive type, having electrical-opticalcrystal such as Lithium-Niobate (LiNbO₃). A pair of duobinary signalsgenerated by duobinary filters 100-1 and 100-2 are applied to electrodesof the MZ modulator after amplification by amplifiers 102-1 and 102-2 upto half wavelength voltage. The numeral 18 is an optical output, and thenumeral 36 is a light source of continuous light which is subject to bemodulated by the MZ modulator 110.

FIG. 22 shows operation of a MZ modulator. FIG. 22( a) shows waveform ofelectrical duobinary signal for driving a MZ modulator. A duobinarysignal is a ternary signal-having three levels +1, 0 and −1 as shown inFIG. 22( b). The transmission factor of a MZ modulator varies sinuouslyas shown in the optical transmission characteristic in FIG. 22( c)depending upon drive voltage which is voltage difference between twoelectrodes of the MZ modulator. When two electrodes are complementarydriven, an undesired chirp in an output optical signal may be zero inprinciple. Therefore, when a D.C. bias voltage (B) is set so that theoptical transmission factor is the minimum as shown in FIG. 22( c), anoptical phase of an optical output switches just when an input voltagecrosses the bias voltage (B), and therefore, an optical duobinary signalwhich has binary intensity waveform is obtained. Although an opticalduobinary signal is a binary intensity signal as shown in FIG. 22( d)and FIG. 22( e) in optically modulated form, it is essentially ternaryduobinary signal if we consider optical phase (0, π), and has theequivalent bandwidth as that of duobinary signal. Therefore, an opticalduobinary signal has the advantages of both binary intensity modulationand ternary duobinary signal, so that demodulation is possible by binaryintensity detection, and narrow-band characteristic of a duobinarysignal is obtained.

FIG. 23 shows a block diagram of a whole duobinary transmission systemincluding an optical duobinary transmitter 120, a transmission line 124,optical amplifiers 122, and a receive system having anoptical-electrical converter 126, a low pass filter 128, a decisioncircuit 130 and an binary data output terminal 132. An optical duobinarytransmitter 120 in FIG. 23 may take the structure as shown for instancein FIG. 21. In a receive side, a signal is demodulated by merelydetecting light intensity as is the case of detection of binaryintensity modulation signal.

FIG. 24 shows actually measured relations between chromatic dispersionand power penalty for 40 Gbit/s optical duobinary signal (white dot) and40 Gbit/s binary NRZ intensity modulation signal (black dot). Thehorizontal axis shows chromatic dispersion value, and the vertical axisshows power penalty for bit error rate (BER) 10⁻⁹. A power penalty isdefined as the increase of receiver sensitivity compared with thatmeasured at the BER of 10⁻⁹ when the chromatic dispersion is 0. Adispersion tolerance is defined so that it is the width of chromaticdispersion value which satisfies the power penalty less than 1 dB. FIG.23 shows that the dispersion tolerance of an optical duobinary signal is200 ps/nm, and the dispersion tolerance of a binary NRZ signal is 95ps/nm, therefore, the former is more than twice as large as the latter.Thus, an optical duobinary signal has the advantage that the restrictionby chromatic dispersion is considerably decreased in high rate signaltransmission which has severe effect of chromatic dispersion.

However, a prior optical duobinary transmission system has thedisadvantage that an optical transmitter is complicated and requestscomplicated signal process. In particular, an optical duobinary signalwhich is generated by a duobinary filter must be amplified up to thelevel which is enough for driving an optical modulator. In general,voltage level requested for driving an optical modulator is severaltimes as high as voltage level for operating a high rate digitalintegrated circuit. Therefore, an amplifier which drives an opticalmodulator is operated in the high power region where an output voltageis apt to saturate.

In case of a binary NRZ intensity modulation system, even if a driveramplifier is operated in high power region where saturation begins, nodegradation of aperture of eye pattern occurs, or waveform is evenshaped by shortening rising time and falling time of waveforms.

However, in case of a ternary duobinary signal, it is essential to keepwaveform itself, therefore, a driver amplifier for driving an opticalmodulator must have fine linearity in gain characteristics.

Thus, if a driver amplifier for driving an optical modulator used in aconventional binary NRZ intensity modulation system is used foramplifying a ternary duobinary signal, a small distortion of waveform isemphasized, and severe inter-symbol interference is generated. Further,it might be possible that an inter-symbol interference is emphasized byreflection between a duobinary filter and an amplifier, and/orreflection between an amplifier and an optical modulator.

FIG. 25( a) shows waveform of duobinary signal (electrical signal) of 40Gbit/s generated by a prior driver, and FIG. 25( b) shows opticalintensity waveforms modulated by said signal. It is noted in FIG. 25that an electrical signal for driving an optical modulator hasasymmetrical pattern in eye apertures between upper eye opening andlower eye opening, and further an optical intensity signal modulated bysaid electrical signal is degraded in waveforms because of inter-symbolinterference although an eye aperture is kept. An inter-symbolinterference degrades receive sensitivity of optical duobinary signal,and dispersion tolerance so that distance for transmission isconsiderably decreased.

SUMMARY OF THE INVENTION

It is an object of the present invention, therefore, to provide a newand improved optical transmitter which overcomes a disadvantage and alimitation of a prior optical transmitter.

It is also an object of the present invention to provide an opticaltransmitter which provides bandwidth restricted optical signal close toideal condition so that optical transmission with long distance, largecapacity, and low cost is obtained.

The above and other objects of the present invention are attained by anoptical transmitter comprising bandwidth restriction means forrestricting bandwidth of input electrical binary signal, anelectrical-optical conversion means for converting electrical signalwhich is bandwidth restricted by said bandwidth restriction means tooptical signal, an amplifier for amplifying an input signal of saidelectrical-optical conversion means so that said input signal has enoughlevel for operating said electrical-optical conversion means, and saidbandwidth restriction means being located between an output of saidamplifier and an input of said electrical-optical conversion means.

In an optical transmitter of the present invention, bandwidthrestriction means which is implemented for instance by a low pass filteris located between an output of an amplifier for driving a modulator andan input of an optical modulator, therefore, said amplifier has only toamplify a binary NRZ signal so that inter-symbol interference problem isavoided even when it operates in saturation region.

In one modification of the present invention, a waveform shaping meanssuch as a low pass filter is implemented by a part of said amplifier orsaid optical modulator, thus, not only an optical transmitter iscompact, but also harmful reflection between an amplifier and a filter,and/or a filter and an optical modulator is avoided so that an opticaltransmitter with high performance is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and attendant advantages ofthe present invention will be appreciated as the same become betterunderstood by means of the following description and accompanyingdrawings wherein;

FIG. 1 is a block diagram of first embodiment of an optical transmitteraccording to the present invention,

FIG. 2 is an example of an optical spectrum generated by the firstembodiment,

FIG. 3 is a block diagram of second embodiment of an optical transmitteraccording to the present invention,

FIG. 4 is an example of an optical spectrum generated by the secondembodiment,

FIG. 5 is a block diagram of third embodiment of an optical transmitteraccording to the present invention,

FIG. 6 is an example of an optical spectrum generated by the thirdembodiment,

FIG. 7 is a block diagram of fourth embodiment of an optical transmitteraccording to the present invention,

FIG. 8 shows waveforms of 40 Gbit/s signal generated by the fourthembodiment,

FIG. 9 shows a curve of dispersion tolerance of 40 Gbit/s signalgenerated by the fourth embodiment,

FIG. 10 is a block diagram of fifth embodiment of an optical transmitteraccording to the present invention,

FIG. 11 is a block diagram of sixth embodiment of an optical transmitteraccording to the present invention,

FIG. 12 is a block diagram of seventh embodiment of an opticaltransmitter according to the present invention,

FIG. 13 shows curves showing decrease of modulation efficiency by phasemismatching in an electrical-optical modulator,

FIG. 14 is a block diagram of eighth embodiment of an opticaltransmitter according to the present invention,

FIG. 15 is a block diagram of ninth embodiment of an optical transmitteraccording to the present invention,

FIG. 16 shows intensity waveforms of a ternary optical duobinary signalgenerated by the ninth embodiment,

FIG. 17 shows binary intensity waveforms of phase inverted opticalduobinary signal generated by the ninth embodiment,

FIG. 18 is a block diagram of tenth embodiment of an optical transmitteraccording to the present invention,

FIG. 19 shows cross section of a Z-cut Lithium-Niobate MZ opticalmodulator,

FIG. 20 shows cross section of an X-cut Lithium-Niobate MZ opticalmodulator,

FIG. 21 is a block diagram of a prior optical duobinary transmitter,

FIG. 22 shows operation of a prior optical duobinary transmitter,

FIG. 23 shows a prior optical transmission system,

FIG. 24 shows curves of dispersion tolerance of 40 Gbit/s generated by aprior optical transmitter, and

FIG. 25 shows waveforms of 40 Gbit/s generated by a prior opticaltransmitter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a block diagram of a first embodiment of an opticaltransmitter according to the present invention. In the embodiment, anoptical transmitter comprises an input terminal 10 which receives anelectrical binary data signal, a binary signal amplifier 12 foramplifying the binary data signal, a bandwidth restriction means 14which is implemented by a low pass filter for restricting bandwidth ofan output signal of the amplifier 12, an electrical-optical conversionmeans 16 for converting electrical signal of an output of the filter 14to optical form, and an optical output terminal 18 coupled with anoutput of the electrical-optical converter 16. The feature in FIG. 1 isthat the bandwidth restriction means 14 is located between the binarysignal amplifier 12 and the electrical-optical conversion means 16,while the bandwidth restriction means in a prior art (FIG. 21) islocated at an input side of an amplifier. Because of the location of thebandwidth restriction means 14 at an output side of the amplifier 12, anundesired distortion generated by the amplifier 12 is removed by thebandwidth restriction means 14, and the resultant optical signal is freefrom undesired distortion. It is a matter of course that an actualtransmitter has many components which are not shown in FIG. 1, forproviding necessary function as an optical transmitter.

The bandwidth restriction means 14 or the filter is equivalent to thecircuit consisting of a one-bit delay which delays an input signal byone bit, and an adder which provides sum of an output of the one-bitdelay and an input signal.

The restriction of bandwidth of an electrical signal by using thebandwidth restriction means 14 restricts the bandwidth of an opticalsignal, and it decreases undesired effect by chromatic dispersion andimproves transmission performance, and the efficiency of the use ofbandwidth by high density wavelength division multiplexing. Anelectrical-optical conversion means 16 may be either a semiconductorlaser in which an optical source is directly modulated, or a combinationof a continuous light source and an external modulator. An externalmodulator may be any type of modulator, for instance, a field absorptiontype modulator using a semiconductor, or a MZ (Mach Zehnder) typemodulator using electrical-optical crystal such as LiNbO₃.

The spectrum of optical signal is designed according to a bandwidthrestriction means 14. As the spectrum of optical signal depends upon aninput signal waveform applied to a binary signal amplifier 12, responsecharacteristic of a binary signal amplifier 12 and an electrical-opticalconverter 16, the response characteristic of a bandwidth restrictionmeans 14 is designed considering those conditions.

When it is desired to suppress signal bandwidth with no inter-symbolinterference, an optical signal from an optical transmitter must satisfythe Nyquist criterion of zero inter-symbol interference.

FIG. 2 shows an ideal signal spectrum which satisfies the Nyquistcriterion I, in which f₀ is clock frequency of a signal, and when bitrate is 40 Gbit/s, f₀=40 GHz. The amplitude spectrum decreases half atfrequency f₀/2 of that of D.C. component, and reaches zero atapproximate f₀. The maximum frequency of the signal spectrum whichsatisfies the Nyquist criterion I distributes between f₀/2 and f₀depending upon shape of spectrum. As it is very difficult to suppressthe maximum frequency of the signal spectrum to f₀/2 in the Nyquistcriterion I, the maximum frequency f₀ of the signal spectrum, as shownin FIG. 2, is allowed so that it is realized by an actual low passfilter.

It is important that the signal waveform shaped by a low pass filter iskept until at least it is transmitted into a transmission line throughconversion into an optical form, therefore, according to the presentinvention, a signal amplifier 12 which is apt to provide inter-symbolinterference is located at an input side of a low pass filter 14, sothat an output of the low pass filter 14 drives directly anelectrical-optical converter 16 to provide an excellent light signalwaveform.

Second Embodiment

FIG. 3 shows a second embodiment of an optical transmitter which usesduobinary code according to the present invention. In order to furthersuppress bandwidth, the bandwidth restriction code such as duobinarycode which satisfies the Nyquist criteron II is used. The bandwidthrestriction means 14 b in the current embodiment is a duobinary filterwhich is a low pass filter having 3 dB cutoff frequency around f₀/4.

FIG. 4 shows a signal spectrum of an ideal duobinary signal. The maximumfrequency of a duobinary signal spectrum is f₀/2, however, as it iscutoff moderately, it may be implemented by using an actual filter.

By the way, as a duobinary filter accompanies code conversion from abinary signal to a ternary signal, the current embodiment has a codeinverter 30 and a precoder 32 before a binary signal amplifier 12.

A code inverter 30 inverts an input code, 0 or 1, to 1 or 0. A precoder32 comprises an exclusive-OR circuit 32 a and a one-bit delay 32 b whichdelays a signal by one bit duration or a period T. Inputs of theexclusive-OR circuit 32 a are an output of the inverter 30 and an outputof the one-bit delay 32 b, and an input of the one-bit delay 32 b is anoutput of the exclusive-OR circuit 32 a.

Due to the presence of a code inverter 30 and a precoder 32, a ternarysignal of an output of a duobinary filter 14 b has a logical signal 1corresponding to high level and low level, and logical signal 0corresponding to intermediate level. Therefore, a receive side maycorrectly demodulate a receive signal by deciding whether a receivesignal is at intermediate level (logical 0) or not (logical 1).

Assuming that an electrical binary signal at an input terminal 10 is(0,1,0,0,0,1,0) as shown in (a), an output (b) of an inverter 30 is aninverted form of (1,0,1,1,1,0,1) an output of a precoder 32 shown in (c)which is in NRZ form of (1,1,0,1,0,0,1), an output of a duobinary filter14 b is shown in (d) which is a ternary duobinary signal having level(0,+1,0,0,0,−1,0), and an optical output of a converter 16 is shown in(e) which has (0,1,0,0,0,1,0) with a phase corresponding to the first 1being opposite to that of the second 1.

As it is essential that a signal shaped by a duobinary filter keeps awaveform itself at least until it is transmitted into a transmissionline through conversion into an optical signal, a signal amplifier whichmay cause inter-symbol interference is located before a duobinaryfilter, so that a distortion generated by an amplifier is removed by aduobinary filter and an output of the duobinary filter drives directlyan electrical-optical converter to provide excellent optical signalwaveform.

Third Embodiment

FIG. 5 shows a third embodiment of an optical transmitter according tothe present invention. A modified duobinary code which satisfies theNyquist criteron III realizes bandwidth suppression similar to aduobinary code. A precoder in the current embodiment is a modifiedprecoder 30′ in which a delay circuit 32 b′ is two bits delay circuitinstead of one bit delay circuit 32 b in FIG. 3. The bandwidthrestriction means in the current embodiment is a band pass filter 14 ccalled a modified duobinary filter. FIG. 6 shows a spectrum of an idealmodified duobinary signal. The maximum frequency of an ideal modifiedduobinary signal spectrum is the same as that of a duobinary signalspectrum. The modified duobinary signal further suppresses D.C.component, so that it has a feature that it is robust to the signalwaveform degradation due to an amplifier with low frequency cutoffcharacteristics. Although a modified duobinary signal is a ternarysignal as is the case of a duobinary signal, it needs a precoder asshown in FIG. 5, which is different from that of FIG. 3 for a duobinarysignal. Because of the presence of a specific precoder, a ternary signalprovided by a modified duobinary filter has a logical 1 corresponding tohigh level and low level, and a logical 0 corresponding to intermediatelevel. Therefore, a receive side may correctly demodulate a receivesignal by deciding whether a receive signal is at an intermediate levelor not. It is essential that a signal shaped by a modified duobinaryfilter keeps waveform itself until it is transmitted into a transmissionline through conversion into an optical signal, therefore, a signalamplifier which may cause inter-symbol interference is located before amodified duobinary filter. Thus, an electrical-optical converter isdirectly driven by an output of a modified duobinary filter so that anexcellent optical waveform is obtained.

Fourth Embodiment

FIG. 7 shows a fourth embodiment of an optical transmitter according tothe present invention. FIG. 7 is similar to the embodiment of FIG. 3,and an electrical-optical converter is implemented by a Mach Zehndermodulator 16. An input signal applied to the Mach Zehnder modulator 16is in differential form, so that a differential distributor 34 isprovided to divide an output of the precoder 32 to two branches. Thenumeral 36 is an optical source which generates coherent light which issupplied to the Mach Zehnder modulator 16. FIG. 7 is an implementationof an optical transmitter which modulates a ternary duobinary signalinto intensity and phase of light, and the logical operation of FIG. 7is the same as that of FIG. 21. The feature of the embodiment of FIG. 7is that duobinary filters (14 b-1, 14 b-2) are located betweenamplifiers (12-1, 12-2) and a MZ light modulator 16. Because of thatfeature, the current embodiment provides optical signal which isfreefrom inter-symbol interference as compared with that of a prioroptical duobinary transmitter in FIG. 21.

FIG. 8 shows measured waveforms of an electrical modulation signal of 40Gbit/s, and a modulated optical signal of 40 Gbit/s. FIG. 8( a) showswaveform of electrical signal which is applied to a Mach Zehndermodulator, and has clear eye aperture on line Q-Q′ having levels 1, 0and −1. FIG. 8( b) is waveform of modulated optical signal having highlevel corresponding to electrical levels +1 and −1, and low levelcorresponding to electrical level 0. When we compare the curves of FIG.8 with those of FIG. 25 which shows prior waveforms, it should be notedthat the waveforms according to the present invention have lessinter-symbol interference and have large eye aperture (on line Q-Q′), ascompared with those of a prior art.

FIG. 9 shows a dispersion tolerance of 40 Gbit/s optical duobinarysignal measured by using the optical transmitter in FIG. 7. It should benoted that the dispersion tolerance which satisfies power penalty lessthan 1 dB is 380 ps/nm, which almost coincides with the expected valueby calculation. Therefore, an optical transmitter of the currentembodiment realizes essentially an ideal optical duobinary signal. Onthe other hand, the dispersion tolerance of a prior optical transmitterfor 40 Gbit/s optical duobinary signal is 200 ps/nm as shown-in FIG. 24.Thus, the present optical transmitter is clearly better than a prioroptical transmitter in view of dispersion tolerance.

Fifth Embodiment

FIG. 10 shows a block diagram of a fifth embodiment of an opticaltransmitter according to the present invention. The feature of thecurrent embodiment is that a binary signal amplifier 12 and a bandwidthrestriction means 14 are integrated into a module 40 so that a connectorbetween a binary signal amplifier 12 and a bandwidth restriction means14 is removed. The current embodiment has the advantages that a numberof components mounted in a transmitter is decreased, and that the effectof reflection between a binary signal amplifier 12 and a bandwidthrestriction means 14 is avoided so that a signal free from inter-symbolinterference is generated.

Sixth Embodiment

FIG. 11 shows a block diagram of a sixth embodiment of an opticaltransmitter according to the present invention. The feature of thecurrent embodiment is that a bandwidth restriction means 14 and anelectrical-optical converter 16 are integrated into a module 42 so thata connector between a bandwidth restriction means 14 and anelectrical-optical converter 16 is removed. The current embodiment hasthe advantages that a number of components mounted in a transmitter isdecreased as is the case of the fifth embodiment, and that the effect ofreflection between a bandwidth restriction means 14 and anelectrical-optical converter 16 is avoided so that a signal free frominter-symbol interference is obtained. Further, when a binary signalamplifier 12 is mounted in the same module 42, an optical transmitterwhich is further compact and stable is obtained.

Seventh Embodiment

FIG. 12 shows a seventh embodiment of an optical transmitter accordingto the present invention. The feature of the current embodiment is thatthe bandwidth restriction means and the electrical-optical converter inthe sixth embodiment is implemented by a MZ light modulator 50 whichuses a Z-cut Lithium-Niobate (LiNbO₃).

Conventionally, for high speed operation, a MZ light modulator usingLithium-Niobate takes shortened electrodes, travelling wave typeelectrodes, and/or specific shape of electrodes. According to thepresent invention, the length and/or shape of electrodes are notdesigned for high speed operation, but designed to satisfy desiredbandwidth restriction performance by using loss in an electrode, andphase mismatching between an electrical modulation signal and an opticalwave which is subject to be modulated, so that a MZ light modulatordoubles as a bandwidth restriction means.

Preferably, the loss in a travelling wave type electrode at f₀/2 isalways larger than the loss at frequency higher than f₀/2, andmodulation efficiency of said Mach Zehnder light intensity modulator atf₀/2 is larger than that at frequency higher than f₀/2, where f₀ isclock frequency of an electrical binary signal.

Preferably, modulation efficiency determined by the phase mismatching ofa Mach Zehnder light intensity modulator at f₀/2 is always larger thanthat at frequency higher than f₀/2, where f₀ is clock frequency of anelectrical binary signal.

In FIG. 12, a numeral 10 is an input terminal of an electrical datasignal, 12 is an electrical amplifier, 50 is an electrical-opticalconverter implemented by a Mach Zehnder light modulator. The numeral 36is an optical source which generates an optical signal subject to bemodulated by the modulator 50, 18 is modulated optical signal. Thenumeral 52 is a D.C. bias terminal which accepts bias potential appliedto the electrode 50 a of the Mach-Zehnder modulator 50, 54 is aninductor, 56 is a terminal resistor, and 58 is a capacitor. The otherelectrode 50 b of the modulator 50 is grounded. An optical beamgenerated by the optical source 36 is separated into two beamstravelling the waveguides p and q, respectively, in the modulator 50 andthen two beams are combined into a single beam so that the beam isamplitude modulated by phase difference between two beams in respectivewaveguides.

FIG. 13 shows the decrease of modulation efficiency caused by phasemismatching between the phase of electrical signal travelling anelectrode, and the phase of optical signal travelling a waveguide. It isunderstood that the modulation efficiency decrease coefficient |r|decreases with vibration when phase mismatching coefficient Δφincreases, according to the following equations.

r=exp[jω _(m) τ_(d)(1−(c/n)c _(m))−1]/jω _(m) τ_(d)(1−(c/n)c _(m))

Δφ=ω_(m) τ_(d)(1−(c/n)c _(m))

where; ω_(m) is angular frequency of electrical signal (rad/s),

τ_(d) is interaction time (s) of an optical signal and an electricalsignal,

c/n is phase velocity (m/s) of an optical wave in an electrical-opticalcrystal,

c_(m) is phase velocity (m/s) of an electrical signal.

As the phase mismatching coefficient Δφ is proportional to angularfrequency ω_(m) of electrical modulation wave, the modulation efficiencydecreases depending upon the increase of the angular frequency ofelectrical modulation wave. Further, the phase mismatching coefficientΔφ is a function of interaction time τ_(d) of electrical modulation waveand optical wave to be modulated, and phase velocity c_(m) of electricalmodulation wave, therefore, the modulation frequency characteristic ofthe modulation efficiency decrease coefficient r is designed accordingto τ_(d) and c_(m). Thus, a MZ light modulator can double as a filter bydesigning modulation efficiency decrease coefficient r properly.

Eighth Embodiment

FIG. 14 shows an eighth embodiment of an optical transmitter accordingto the present invention. The feature of the present embodiment isan-optical duobinary transmitter which has a dual electrode type MZlight modulator 60 having LiNbO₃, and each of said electrodes (61-1,61-2) is used as a duobinary filter. A precoded binary NRZ signal indifferential form is applied to a pair of electrodes (61-1, 61-2) of theMZ light modulator 60, so that the polarity of a signal applied to afirst electrode 61-1 is opposite to the polarity of a signal applied toa second electrode 61-2. As the electrodes function as a duobinaryfilter, the electrodes are designed so that the duobinary filter has 3dB cut-off frequency around the frequency of ¼ of a signal clockfrequency. The phase change in the modulated optical signal haswaveforms of a ternary duobinary signal, and the MZ light modulatorprovides an optical duobinary signal in which the light intensity is themaximum for the maximum phase change and the minimum phase change, thelight intensity is the minimum around the midpoint of the optical phasechanges, and the phase of the optical signal of the maximum intensityfor the maximum phase is opposite to that for the minimum phase.

Ninth Embodiment

FIG. 15 shows a ninth embodiment of an optical transmitter according tothe present invention. The feature of the current embodiment is that thetravelling direction of electric modulation wave propagating on anelectrode is opposite to the travelling direction of optical wavepropagating in an optical waveguide in Lithium-Niobate crystal so thatphase mismatching condition is generated to provide bandwidthrestriction characteristic in electrodes. Other portions in FIG. 15 arethe same as those in FIG. 14. In FIG. 15, optical wave generated by alight source 36 propagates in the figure from left to right, whileelectric wave of an output of amplifiers 12-1, and 12-2 propagate in thefigure from right to left. The opposite travelling direction means thata sign of light velocity c/n in a waveguide and a sign of an electricmodulation wave velocity c_(m) are opposite to each other in thedefinition of the phase mismatching coefficient Δφ in FIG. 13,therefore, the value (1−(c/n)c_(m)) is larger than 1, and the phasemismatching coefficient Δφ is large.

In order to confirm the effect of the ninth embodiment, we measuredoptical intensity waveforms obtained by using a Lithium-Niobate MZoptical modulator in which an input electrical wave travels in oppositedirection to that of an optical output. The modulator has 10 GHzbandwidth when an input electrical wave travels in the same direction asthat of an optical output. The amplitude of the electrical modulationsignal and the bias voltage of the MZ optical modulator were set so thatlight intensity can be observed as a ternary duobinary signal, in orderto observe that the bandwidth restriction caused by phase mismatchingconverts a binary NRZ signal into a ternary duobinary signal. FIG. 16shows an eye pattern of the measured ternary duobinary signal having bitrate 3.3 Gbit/s. An eye aperture opens along a line R-R′. It isunderstood visually in FIG. 16 that the bandwidth is restricted by thephase mismatching between electrical modulation wave and light wavewhich is subject to be modulated, and a binary NRZ signal is convertedinto a ternary duobinary signal. A phase inverted optical duobinarysignal is obtained by modifying only bias voltage of a MZ modulator inFIG. 16, and FIG. 17 shows the intensity waveforms of said phaseinverted optical duobinary signal. It is understood in FIG. 17 that aneye aperture is kept as a binary signal. The drive voltage of the MZmodulator in FIG. 17 is half of the drive voltage requested for ordinaryoptical duobinary modulation, since FIG. 17 is intended to measureoptical intensity waveforms for confirming the operational principle ofthe invention, therefore, an inter-symbol interference on high levelside is large. That inter-symbol interference may be suppressed when anoptical modulator is driven with a regular drive voltage.

Tenth Embodiment

FIG. 18 shows a tenth embodiment of an optical transmitter according tothe present invention. In the current embodiment, an X-cutLithium-Niobate MZ optical modulator is used, but not Z-cut.

Lithium-Niobate is uni-axis crystal, having the highest modulationefficiency when electric field is applied in Z-axis direction,therefore, Z-cut crystal is usually used in an ordinary electric-opticalmodulator.

FIG. 19 shows cross section (along A-A′ in FIG. 15) of Z-cutLithium-Niobate optical modulator 70. The numeral 71 is a substrate ofZ-cut Lithium-Niobate, on which, through a silica buffer layer 72,ground electrodes 73-1, 73-2, 73-3, and a pair of signal electrodes 74-1and 74-2 are deposited. Under the signal electrodes 74-1 and 74-1, apair of waveguides 75-1 and 75-2 with titanium (Ti) diffused areprovided. In a Z-cut Lithium-Niobate optical modulator, electric field Eis applied perpendicular to a substrate surface in an optical waveguideas shown by an arrow 76, and a pair of electrodes 74-1, 74-2 must bedriven complementary so that no chirp is provided, such as an opticalduobinary modulation.

FIG. 20 shows cross section (along B-B′ in FIG. 18) of an X-cutLithium-Niobate optical modulator 80 used in the embodiment of FIG. 18.The numeral 81 is a substrate of X-cut Lithium-Niobate, on which,through a silica buffer layer 82, ground electrodes 83-1 and 83-2, and asignal electrode 84 are deposited. In regions between signal electrodesand a ground electrode waveguide 85-1 and 85-2 with titanium diffusedare provided. In an X-cut Lithium-Niobate optical modulator, an electricfield E is applied parallel to a substrate surface in an opticalwaveguide, as shown by an arrow 86. Therefore, when a signal electrode84 is located at the center of two waveguides of a MZ modulator, and apair of ground electrodes 83-1 and 83-2 are located on both ends of thesubstrate symmetrically to the signal electrode 84 (see FIG. 18),opposite electric fields are always applied to each of the waveguides.Therefore, a chirp is zero in principle when a single electrode MZmodulator is used. The current embodiment makes those electrodes havingthe bandwidth restriction performance, thus, an optical duobinary signalis generated with a simple structure of a MZ modulator.

As described above, according to the present invention, waveform shapingmeans such as a low pass filter is located between an electricalamplifier which drives an optical modulator, and an optical modulator,so that the electrical amplifier has only to amplify an electricalbinary NRZ signal. Thus, a severe inter-symbol interference problemcaused by the operation of the electrical amplifier in gain saturationregion is avoided. Further, when a waveform shaping means such as a lowpass filter is integrated with a driver amplifier or an opticalmodulator, an optical transmitter may be compact and further harmfulreflection is decreased between an amplifier and a filter, or between afilter and an optical modulator. Therefore, a bandwidth restrictedoptical signal close to ideal condition is generated. This is beneficialto make distance long, capacity large and cost low in an opticalcommunication system.

From the foregoing, it is now apparent that a new and improved opticaltransmitter has been found. It should be understood of course that theembodiments disclosed are merely illustrative and are not intended tolimit the scope of the invention. Reference should be made, therefore,to the appended claims to indicate the scope of the invention.

1-17. (canceled)
 18. An optical transmitter comprising; precoding meansfor providing an output which is the same as the previous output when aninput binary digital signal is 0, and an output which differs from theprevious output when an input digital signal is 1, bandwidth restrictionmeans for generating a ternary duobinary signal by restricting thebandwidth of a binary digital signal output from the precoding means,and a Mach Zehnder light intensity modulator driven by an output signalof the bandwidth restriction means, wherein the Mach Zehnder lightintensity modulator has a substrate of X-cut Lithium-Niobate, on which asignal electrode, a first ground electrode and a second ground electrodeare provided, the signal electrode is located between a first opticalwaveguide and a second optical waveguide which constitute the MachZehnder light intensity modulator, the first ground electrode is locatedopposite side of the signal electrode relative to the first opticalwaveguide, the second ground electrode is located opposite side of thesignal electrode relative to the second optical waveguide, andelectrical fields between the signal electrode and the first/secondground electrodes cross the first/second optical waveguides parallel toa surface of the substrate.
 19. An optical transmitter comprising;precoding means for providing an output which is the same as theprevious output when an input binary digital signal is 0, and an outputwhich differs from the previous output when an input digital signal is1, and a Mach Zehnder light intensity modulator driven by an outputsignal of the precoding means, wherein the Mach Zehnder light intensitymodulator has a substrate of X-cut Lithium-Niobate, on which a signalelectrode, a first ground electrode and a second ground electrode areprovided, the signal electrode is located between a first opticalwaveguide and a second optical waveguide which constitute the MachZehnder light intensity modulator, the first ground electrode is locatedopposite side of the signal electrode relative to the first opticalwaveguide, the second ground electrode is located opposite side of thesignal electrode relative to the second optical waveguide, andelectrical fields between the signal electrode and the first/secondground electrodes cross the first/second optical waveguides parallel toa surface of the substrate, wherein the signal electrode is a travelingwave type electrode, and bandwidth of optical output of the Mach Zehnderlight intensity modulator is restricted because of loss of the travelingwave type electrode.
 20. An optical transmitter comprising; precodingmeans for providing an output which is the same as the previous outputwhen an input binary digital signal is 0, and an output which differsfrom the previous output when an input digital signal is 1, and a MachZehnder light intensity modulator driven by an output signal of thepreceding means, wherein the Mach Zehnder light intensity modulator hasa substrate of X-cut Lithium-Niobate, on which a signal electrode, afirst ground electrode and a second ground electrode are provided, thesignal electrode is located between a first optical waveguide and asecond optical waveguide which constitute the Mach Zehnder lightintensity modulator, the first ground electrode is located opposite sideof the signal electrode relative to the first optical waveguide, thesecond ground electrode is located opposite side of the signal electroderelative to the second optical waveguide, and electrical fields betweenthe signal electrode and the first/second ground electrodes cross thefirst/second optical waveguides parallel to a surface of the substrate,wherein the signal electrode is a traveling wave type electrode, andbandwidth of optical output of the Mach Zehnder light intensitymodulator is restricted by using mismatching of phase velocity ofelectric wave propagating on the traveling wave type electrode andoptical wave propagating in an optical waveguide having refractive indexdepending upon electrical field generated by said electric wave.